Canine Monocytic Ehrlichiosis Paul R Earl Facultad de Ciencias Biol

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Canine Monocytic EhrlichiosisPaul R
EarlFacultad de Ciencias BiológicasUniversidad
Autónoma de Nuevo LeónSan Nicolás, NL, Mexico
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INTRODUCTIONTHIS LECTURE IS based
mainly on the excellent work of T. Waner and S.
Harrus that is found in Recent Advances in Canine
Infectious Diseases, 2000, Carmichael L., Editor,
Internatl Vet Information Service, Ithaca, NY
(www.ivis.org) The etiologic agent of canine
monocytic ehrlichiosis (CME), Ehrlichia canis is
a small pleomorphic gramnegative coccoid that
parasitizes circulating monocytes
intracytoplasmically in clusters of organisms
called morulae. Ehrlichia is very closely related
to the intraerythrocyric bacterium Anaplasma so
that in the future PERHAPS this disease will be
called canine anaplasmosis.
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PATHOGENESISEhrlichia canis is transmitted by
the brown dog tick Rhipicephalus sanguineus and
by Dermacentor variabilis. Larvae and nymphs have
been shown to become infected while feeding on
infected dogs.
Male and female brown dog ticks Rhipicephalus
sanguineus.
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A male dog tick of Dermacentor variabilis.
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The distribution of CME is related to the
distribution of the vector R. sanguineus and has
been reported to occur in Asia, Africa, Europe
and the Americas. Seroprevalence of E. canis
antibodies in dogs in Zimbabwe, Egypt and Israel
have been shown to be 42, 33 and 30
respectively. The pathogenesis of CME involves
an incubation period of 8-20 days, followed by
acute, subclinical and sometimes chronic phases.
German shepherd dogs (GSD) tend to develop the
severe chronic phase of the disease more often
than other breeds.
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CLINICAL PRESENTAIONNaturally occurring
CME may be manifested by a wide variety of
clinical signs. Large variations in clinical
signs have been reported for different studies
and have been proposed to be due to a number of
factors, including differences in pathogenicity
between strains of the ehrlichia, breed of dog,
coinfections with other tick transmitted diseases
and the immune status of the dog. There is no
predilection for age or sex in infection with E.
canis and all breeds may be infected. However,
the GSD seems to be more prone to develop
clinical CME.
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Ocular signs are not uncommon and include
anterior uveitis corneal opacity (edema and
deposition of cellular precipitates), hyphema,
tortuous retinal vessels and focal chorioretinal
lesions consisting of central pigmented spots
with surrounding areas of hyperreflectivity.
Subretinal hemorrhages, resulting in retinal
detachment and blindness may occur. Other
clinical signs may include vomiting, serous to
purulent oculonasal discharge, lameness, ataxia
and dyspnea.
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HEMATOLOGYThrombocytopenia
is the most common and consistent hematological
finding in acute CME. A concurrent significant
increase in the mean platelet volume is also
usually seen reflecting active thrombopoiesis.
Mild leukopenia and mild anemia (usually
normocytic, normochromic, non-regenerative)
commonly occur in the acute stage of the disease.
Mild thrombocytopenia is a common finding in the
subclinical stage of the disease. A
decline in the neutrophil counts may occur.
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The blood cells above in this illustration are
stained with Giemsa as usual. A morula of E.
canis is seen directly under the monocytes
nucleus. Also 2 small coccoids are seen in the
cytoplasm of the monocyte.
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Mourula in a monocyte that can dissolve into
several cocci. Giemsa.
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BIOCHEMICAL FINDINGSHypoalbuminemia,
hyperglobulinemia and hypergammaglobulinemia are
the principal biochemical abnormalities seen in
dogs infected with CME. Serum protein
electrophoresis usually reveals polyclonal
gammopathy. Infected dogs on rare occasions may
present with monoclonal gammopathy, which may be
misdiagnosed as paraproteinemia. Pancytopenic
dogs reveal significantly lower concentrations of
total protein, total globulin and gammaglobulin
concentrations as compared to nonpancytopenic
dogs.
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DIAGNOSISThe majority of CME
cases occur in endemic areas during the spring
and summer months when the tick population is
most active. Diagnosis of CME is based on
anamnesis, clinical presentation, clinical
pathological findings and confirmed by laboratory
tests. Owners may report previous tick
infestations or a recent visit to an endemic
area.Diagnosis of CME is confirmed by
visualization of the morulae in circulating
monocytes, detection of increased serum
antibodies to E. canis, or by the demonstration
of E. canis DNA by polymerase chain reaction
(PCR).
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Dot-ELISA tests have recently been developed for
use in the clinic. These tests require the
minimum of equipment and will make serologic
diagnosis of CME available on a wider basis. This
should prove to be an invaluable in-clinic aid in
the serodiagnosis of CME.When assessing IFA
antibody titers for E. canis in dogs it is
essential that the diagnostician take in account
the range of cross-reactivities that may also
confound the diagnosis. In areas endemic to other
Ehrlichia species, crossreactivity between E.
canis and E. ewingii, E. equi or E. risticii
should be taken into consideration.
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Crossreactivity between E. canis, Neorickettsia
helminthoeca (the etiologic agent of salmon
poisoning disease) also has been documented.
There is no serologic crossreaction between E.
canis and E. platys. Due to the confusion caused
by crossreacting infections, it is desirable to
test sera against a number of agents.Generally a
4-fold difference between the titers of
antibodies to the different antigens is
considered to infer etiology, where patients
react to a number of antigens. The possibility of
multiple tickborne infections may confound the
use of serological testing. Coinfection with E.
canis, E. chaffeensis and E. ewingii, E. equi, E.
platys, Rickettsia species, Bartonella species
and Babesia canis has been documented in a kennel
of heavily tick-infested dogs.
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Diagnosis of the subclinical disease should be
based on anamnesis, geographic location of the
dog, persistent antibody titers to E. canis, mild
thrombocytopenia and hypergammaglobulinemia. The
diagnosis of the disease at this stage is a
challenge to the practicing veterinarian. The
importance of early diagnosis lies in the
relatively good prognosis before some of the dogs
enter the chronic phase, at which stage the
prognosis is grave. The chronic disease is the
end-stage of the disease process and the
diagnosis is based on the anamnesis, the typical
severe pancytopenia, presence of antibody titers
to E. canis, serum hypergammaglobulinemia and
lack of response to doxycycline therapy. This
stage is usually easier to diagnose.
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TREATMENTDoxycycline at a dose
of 10 mg/kg once daily (or 5 mg/kg twice daily)
for a period of 3 weeks at least, is the
treatment of choice for acute CME. Short term
treatment with doxycycline (10 mg/kg, once
daily, for 7 days) has been shown to result in
failure, while 10 days doxycycline treatment has
shown success. Ten days treatment may not be
enough in all cases. Most dogs suffering acute
CME respond to treatment and show clinical
improvement within 24-72 hours. Dogs in the
subclinical stage may need a prolonged treatment
compared to dogs suffering the acute stage.
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Other drugs with known efficacy against E. canis
include tetracycline hydrochloride (22 mg/kg, q 8
hrs), oxytetracycline (25 mg/kg, q 8 hrs),
minocycline (20 mg/kg, q 12 hrs) and
chloramphenicol (50 mg/kg, q 8 hrs). In a recent
report it has been shown that oral enrofloxacin
(5 or 10 mg/kg q 12h for 21 days) was not
effective in elimination of the rickettsia from
experimentally infected dogs.The use of
immunosuppressive doses of glucocorticosteroids
in the treatment of the acute stage of CME can be
considered. However, as no clinical studies have
been performed to prove the efficacy of steroids
in the treatment of CME, these steroids should be
used with caution.
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Treatment of the chronic severe form of the
disease is unrewarding and the prognosis of these
pancytopenic dogs is grave. Only one report
described a successful treatment of a dog with
severe chronic CME, using a combination of
hematopoietic growth factors (recombinant human
granulocyte colony stimulating factor and
recombinant human erythropoietin) low dose
vincristine, doxycycline and a long course of
glucocorticoid therapy. However, the use of
growth factors in the treatment of chronic
ehrlichiosis has not been proven effective and
requires much further investigation.
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PROPHYLAXIS AND THE FUTURETo date, no
effective E. canis vaccine has been developed and
tick control remains the most effective
preventive measure against infection. Insecticide
impregnated dog collars are an outstanding
success, and protect dogs against other paramount
diseases such as leishmaniasis. In endemic areas,
low dose oxytetracycline treatment (6.6 mg/kg)
once daily has been suggested as a prophylactic
measure for CME. Recently this method has been
used with success by the French army on dogs in
Senegal, Ivory Coast and in Djibouti, where dogs
were treated prophylactically with 250 mg per day
oxytetracycline that was given orally.
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Within a decade perhaps all of the species of
the family Anaplasmatacea will be assigned their
proper taxonomic values. Does one ehrlichia
prefer monocytes, and another granulocytes? How
E. canis relates to human anaplasmosis, formerly
ehrlichiosis is problematic. Recent genetic
nucleotide sequencing of 16S rRNA genes, groESL
and sequencing of surface protein genes indicate
that the family of Anaplasmataceae was
incorrectly ordered until corrected by Dumler and
others (Internatl J Sys Evol Microbiol, 2001, 51
21452165). They found 4 clades 1/ Anaplasma,
including the Ehrlichia phagocytophila group, E.
platys and E. bovis), 2/ E., including E.
ruminantium, 3/ Wolbachia and 4/ Neorickettsia,
including E. sennetsu and E. risticii ).
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A. phagocytophila, Ehrlichia equi and the human
granulocytic ehrlichiosis (HGE) agent were very
similar, equal perhaps to one species. The tribes
Ehrlichieae and Wolbachieae were transferred to
the family Anaplasmataceae and the Rickettsiaceae
were eliminated. These findings do of course
follow DNA nucleotide results and thus dictate
nomenclature and other changes that should be
recognized by veterinary practioners now. This
PCR work is still under construction therefore
the eventual disposition of E. canis at the
species or subspecies level and its finepoint
phylogenetic position lie in the future.